[Zope] ZClass

Hi, Just a little question: Yesterday, I was working with ZClasses. While making one of my own, I saw that Zope asked for a value to enter per property. Why does Zope asks for a value to enter for a new property when you're making a class? I understand that you've to enter a value when you're making a Class instance? But while defining a class??? Is this meant as a default value?? This is particulary strange when working with a propery as date. What to do, if you don't want to give a default value? Zope always complains that you have to give a value. Please, help to clear my mind Tom.